1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to apparatus and methods for laser shock peening laser with a single head laser.
2. Description of Related Art
Laser shock peening (LSP) or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high and low power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477 entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957 entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,756,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No. 5,591,009 entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,531,570 entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447 entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328 entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee.
High energy laser beams, from about 20 to about 50 Joules, or low energy laser beams, from about 3 to about 10 Joules, have been used and other levels are contemplated. See, for example, U.S. Pat. No. 5,674,329 (Mannava et al.), issued Oct. 7, 1997 (LSP process using high energy lasers) and U.S. Pat. No. 5,932,120 (Mannava et al.), issued Aug. 3, 1999 (LSP process using low energy lasers). Low energy laser beams can be produced using different laser materials such as neodymium doped yttrium aluminum garnet (Nd YAG), Nd:YLF, and others. Low energy laser shock peening was developed in an effort to reduce the cost of the laser apparatus, reduced maintenance expenses, and reduce the cost of the laser shock peening process. However, these laser machines are still more complex and expensive as compared to conventional laser drilling machines. Laser drilling machines typically have a single laser.
It is known that large laser pulses, on the order of 20 to 50 Joules (J) per pulse with a leading edge temporal rise time of 5 to 15 nanoseconds and a full width half maximum pulse duration of 20 to 50 nanoseconds can be generated. To create these large laser pulses, the laser generator becomes complex and physically large, incorporating six to over a dozen glass laser rods or heads in order to deliver the energy required for LSP. These lasers are limited to generating a pulse every few seconds to around 2 pulses per second. Lower energy lasers that deliver pulse energies on the order of 5 to 10 Joules enable alternative laser technologies such as Nd:YAG rod crystals to be used. These can deliver the same temporal characteristics at lower energies but at a faster pulsing rate of around 10 Hertz.
In either case, glass at 50 Joules or YAG at 5 Joules, the number of laser heads or rods required to generate the noted energies makes these systems complex, expensive, expensive to maintain and have lower availability for LSP production. Thus, it is highly desirable to have a laser shock peening apparatus and method that incorporates a single head laser for laser shock peening (LSP).